Underwater sound sensing transducers, such as hydrophones and the like, normally employ elements sensitive to sound pressure waves created in the water by a sound source, such as an explosion, a propeller or the like. Underwater sound sensing devices of the motion detecting type known as seismic geophones have been successfully employed to sense the presence and direction of underwater sound pressure waves. A geophone contains a suspended mass within a housing and a sensor arrangement to sense any relative movement in a particular direction between the mass and the housing. The sensor generates corresponding electrical signals which are transmitted to a remote location for analysis to obtain information about the sound source. The sensor is usually a magnetic or a piezoelectric type as is well known in the art. For the purposes of this discussion the geophones are assumed to be positioned to sense the horizontal component of pressure waves. Typically, they are cylindrical and respond to sound components along their axes.
The sensitivity of these sound detecting devices depends on the mass of the seismic body, the mass of the housing, and the frontal area of the housing. A small housing mass, of course accelerates more readily than a large mass to produce a sensible displacement. On the other hand a body with a large frontal area is affected more by a sound wave. In order for the housing to undergo an acceleration to produce a displacement, a force must be applied to the housing to react with the mass of the housing. The force is developed by the sound pressure wave gradient across the housing acting on the effective frontal area of the housing. The frontal area may be considered to be the area of the projection of the housing onto a plane perpendicular to the horizontal component of the sound. Thus it is desirable to maintain a substantial frontal area while minimizing the mass of the housing to increase the sensitivity.
The prior U. S. Pat. Nos. 3,720,909 to Sikora and 3,980,985 to Dale reveal hydrophones of the kind just described. Sikora teaches the desirability of minimizing the mass. These systems have cylindrical housings with vertical axes and include buoyancy chambers above and below the geophones which add to the mass of the housing as well as to the frontal area. The cylindrical housing design has proven to be effective for use at moderate depths. A reasonable wall thickness can be used for the housing since pressure on the housing is not extreme at moderate depths. On the other hand, deep submergence hydrophones encounter pressures of several thousand pounds per square inch. The wall thickness cannot be increased enough to withstand such pressures without unacceptable increase in mass and consequent loss in sensitivity.